Re: [RFC PATCH 1/2] mm: always call rmap_walk() on locked folios

[Barry Song <21cnbao@gmail.com>]

On Mon, Sep 8, 2025 at 12:50 PM Lokesh Gidra <lokeshgidra@google.com> wrote:
>
> Prior discussion about this can be found at [1].
>
> rmap_walk() requires all folios, except non-KSM anon, to be locked. This
> implies that when threads update folio->mapping to an anon_vma with
> different root (currently only done by UFFDIO MOVE), they have to
> serialize against rmap_walk() with write-lock on the anon_vma, hurting
> scalability. Furthermore, this necessitates rechecking anon_vma when
> pinning/locking an anon_vma (like in folio_lock_anon_vma_read()).
>
> This can be simplified quite a bit by ensuring that rmap_walk() is
> always called on locked folios. Among the few callers of rmap_walk() on
> unlocked anon folios, shrink_active_list()->folio_referenced() is the
> only performance critical one.

As I understand it, shrink_inactive_list() also invokes folio_referenced().
Shouldn’t it be called just as often as shrink_active_list()?

>
> shrink_active_list() doesn't act differently depending on what
> folio_referenced() returns for an anon folio. So returning 1 when it
> is contended, like in case of other folio types, wouldn't have any
> negative impact.

A complaint was raised that the LRU could become slightly disordered:
https://lore.kernel.org/linux-mm/20240219141703.3851-1-lipeifeng@oppo.com/

We can re-test to confirm if this is the case.

Thanks
Barry